1. Field of the Invention
The present invention relates to a booster apparatus of a vacuum type used in a brake device of an automobile.
2. Related Background Art
FIG. 8 shows one example of conventional booster apparatuses of a vacuum type. The booster apparatus includes a housing 1 having therein a constant pressure chamber 4 and a variable pressure chamber 5 which are divided by a diaphragm 3 supported by a power piston 2. Negative pressure generated by an internal combustion engine (not shown) is introduced into the constant pressure chamber 4 through an intake pipe 6. The booster apparatus also includes a generally cylinder-shaped valve body 7 having at the outer side thereof annular grooves 2a and 3a. The inner ends of the power piston 2 and the diaphragm 3 are fixedly fitted into the annular grooves 2a and 3a, respectively.
The valve body 7 includes a large diameter cylinder portion 8 mounted on the power piston 2 and the diaphragm 3 and a small diameter cylinder portion 9. The small diameter cylinder portion 9 is integrally formed with the large diameter cylinder portion 8. The rear end portion of the small diameter cylinder portion 9 extends through the rear of the housing 1 outwardly of the housing. An appropriate portion between the rear of the housing 1 and the small diameter cylinder portion 9 is tightly sealed to prevent the variable pressure chamber 5 from being brought into communication with the atmosphere through between the rear of the housing 1 and the small diameter cylinder portion 9. The interior of the valve body 7 communicates with the atmosphere.
An input shaft 10 connected to a brake pedal (not shown) extends into the small diameter cylinder portion 9. A plunger 11 moving in interlocking relation with the input shaft 10 is provided at the leading end portion of the input shaft 10. The valve body 7 includes therein a plunger supporting slide hole portion 12. The plunger 11 slides along the slide hole portion 12. The valve body also includes at its leading side a recess 13 which has a larger inner diameter than the slide hole portion 12. The recess 13 is adjacent to the slide hole portion 12. The base portion 15 of an output shaft 14 and the whole of an elastic reaction disk 16 are inserted into the recess 13. The diameter of the base portion 15 of the output shaft 14 is substantially identical to (or slightly smaller than) the inner diameter of the recess 13. The diameter of the reaction disk 16 is also substantially identical to (or slightly smaller than) the inner diameter of the recess 13.
The valve body 7 includes an air passage 17 for communication of the variable pressure chamber 5 with the inside of the valve body 7 and a negative pressure passage 18 for communication of the constant pressure chamber 4 with the inside of the valve body 7. Provided on the inside of the valve body 7 is a valve member 21. An air valve 19 includes portions of the valve member 21 and the plunger 11. A vacuum valve 20 includes portions of the valve member 21 and the valve body 7. The valve member 21 is biased toward the output shaft 14 by a valve spring 22 provided between the valve member 21 and the input shaft 10. The air valve 19 and the vacuum valve 20 are opened and closed in response to the movement of the input shaft 10, the plunger 11 and the valve body 7 relative to each other. A spring 40 is provided between the small portion 9 and the input shaft 10. A diaphragm return spring 41 is provided in the housing 1 for biasing the valve body 7 toward the input shaft 10.
In the initial state as shown in FIG. 8 (during which the engine is in operation and the brake pedal is not in operation), the constant pressure chamber 4 is the same in pressure (negative pressure) as the variable pressure chamber 5, and both the air valve 19 and the vacuum valve 20 are in a closed position. The application of at least a force of F.sub.1 (see FIG. 9) pushing the input shaft 10 then moves the plunger 11 forwardly of the valve member 21, thereby opening the air valve 19 with the vacuum valve 20 being in the closed position. The opening of the air valve 19 allows the air to flow into the variable pressure chamber 5 through the air passage 17, thereby causing a pressure difference between the constant pressure chamber 4 and the variable pressure chamber 5. Thus, this provides the output shaft 14, through the valve body 7 and the reaction disk 16, with the force to propel it forwardly (or a jumping value indicated in the output area "A" of FIG. 9). If a force less than F.sub.1 is applied to the input shaft 10, a force pushing the output shaft 14 forwardly is not created. The application of force less than F.sub.1 to the input shaft 10 cannot compress the springs 22 and 40 to move the input shaft 10 and the plunger 11 forwardly to open the air valve 19.
Application of a force between F.sub.1 and F.sub.3 (see FIG. 9) to the input shaft 10 to open the air valve 19 causes the above pressure difference to be generated, thereby creating a force for propelling the valve body 7 forwardly. The valve body propelling force is transmitted to the output shaft 14 through the reaction disk 16. The value of the output force for pushing the output shaft 14 is then that of the input force for pushing the input shaft 10 multiplied by a force-boosting ratio of the booster apparatus. For example, the value Fo of the output force for pushing the output shaft 14 becomes the same value as that shown in the output area B of FIG. 9. The force-boosting ratio is determined by a ratio between the contact area of the reaction disk 16 (or the base portion 15 of the output shaft 14) with the valve body 7 and the plunger 11 and the contact area of the plunger 11 with the reaction disk 16. As mentioned above, in response to the transmission of the input shaft-pushing force to the output shaft 14, the reaction from the output shaft 14 is transmitted to the brake pedal through the input shaft 10, thereby enabling a driver to receive a sense of the reaction via the brake pedal when pushing the brake pedal.
If a force of F.sub.3 is applied to the input shaft 10, the pressure in the variable pressure chamber 5 becomes identical to the atmospheric pressure so that the pressure difference between the chambers 4 and 5 is maximized. Thus, a force above F.sub.3 from the input shaft 10 is transmitted to the output shaft 14 by the plunger 11 being pressed against the reaction disk 16.
In the booster apparatus of a vacuum type, the introduction of the atmospheric pressure to the variable pressure chamber 5 causes the pressure difference between the constant pressure chamber 4 and the variable pressure chamber 5. The force of the pressure difference moves the power piston 2 forwardly, and thereby the output shaft 14 generates a boosted thrusting force.
It is necessary for a booster apparatus of a vacuum type to be able to generate a strong force in a case of an emergency. In order to do this, it is necessary to increase the jumping valve as shown in the output area A of FIG. 9 or the force-boosting ratio. In the conventional booster apparatus of a vacuum type, since the force-boosting ratio is constant, increasing the jumping valve or force-boosting ratio causes an excessive braking force to be applied even if the brake pedal is pressed down lightly, causing the brakes to be slammed on. In order to avoid this happening, it is necessary for a driver to operate the brake pedal carefully, even in normal driving conditions, resulting in a driver feeling uncomfortable, and ill at ease.